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These tokens are generated by the app for every action -- like adding friends or sending snaps -- in order to avoid sending the password every time. However, since past tokens don't expire, they can be reused from different devices to send commands through the Snapchat API (application programming interface).

"I'm able to use a custom script I've created to send snaps to a list of users from several computers at the same time," Sanchez said. "That could let an attacker send spam to the 4.6 million leaked account list in less than one hour."

However, in addition to spamming a large number of users, the new issue discovered by Sanchez can also be used to attack a single user by sending him hundreds or thousands of snaps using unexpired tokens.

When this attack is performed against a user who uses Snapchat on an iPhone his device will freeze and the OS will eventually reboot itself, Sanchez said.

The researcher demonstrated the attack against the iPhone of a reporter from the Los Angeles Times with his approval by sending 1,000 messages to the reporter's Snapchat account within five seconds. A video of the demonstration was also posted on YouTube.

"Launching a denial-of-service attack on Android devices doesn't cause those smartphones to crash, but it does slow their speed," Sanchez said. "It also makes it impossible to use the app until the attack has finished."

There is a limiting factor to this attack: the default privacy setting in Snapchat that only allows accounts in a user's friends list to send him snaps, meaning the attacker would first have to convince the targeted user to add him as a friend. According to Snapchat's documentation, sending a snap to a user without being in his list of friends will result in the user receiving a notification so they can add back the sender.

Users who changed their account's default privacy setting so they can receive snaps from anyone would be directly exposed to the attack described by Sanchez.

Snapchat did not immediately respond to a request for comment.

Sanchez said via email that he didn't report the issue to Snapchat before disclosing it publicly because he feels the company has a poor attitude toward security researchers based on how it handled previous vulnerabilities reported to it. In December a security research outfit called Gibson Security published an exploit that allowed attackers to match phone numbers to Snapchat accounts after claiming that the company didn't fix the underlying vulnerability for four months.

According to Sanchez, the problem disclosed by him still hadn't been fixed Saturday, but two accounts and a VPN IP address that he used for testing had been banned. Instead of banning the accounts of a researcher who has no interest in attacking real users and doesn't even use the service, the company should work on improving the security of their application, Sanchez said.

The researcher believes preventing this issue would require an easy fix on the server side. He doesn't know why the OS crashes on iPhones, but he suspects that it has something to do with the Push Notification system that iOS devices use to receive notifications from third-party applications. The research into that aspect continues, he said.